1. Field of the Invention
The present invention relates to an isoelastic open knit useful in particular in parietal and/or visceral surgery, but which can also be used in other areas of surgery.
2. Description of Related Art
The present invention will be described more particularly by referring to a prosthetic knit of said type, intended to be used for the repair of hernias or eventrations. The abdominal wall consists of muscles which are physiologically deformable in all directions, and therefore an ideal reinforcement must follow these physiological deformations whilst providing additional strength to a weakened wall. Even though there is a lack of published data, the lower limit of tensile strength of a prosthetic knit for the repair of hernias or eventrations is in the region of 10 kg, when tested according to the NFG 07-001 standard.
In the present application, the expressions given below have the following meanings:
the term xe2x80x9cfabricxe2x80x9d means an assembly of yarns, obtained in particular by knitting and/or weaving;
the term xe2x80x9cbiocompatible polymer materialxe2x80x9d means a material, or its possible products of degradation in a biological medium, which induces no effect which is toxic or contrary to the sought effect, once implanted in the body.
For a long time, surgeons have used flat prosthetic fabrics, that is to say two-dimensional ones, to repair or replace any part of the body, such as an aponeurosis or a muscular wall, destroyed or damaged, for example as a result of a traumatism. Consequently there are now a large number of such prosthetic fabrics manufactured or obtained according to various processes, for example weaving, knitting or molding, and which have often been designed to carry out specific functions within the body in which they are implanted. Such prosthetic fabrics are described for example in documents U.S. Pat. No. 5,569,273, WO-A-96/03091 and EP-A-0797962, which divulge the knitting of monofilaments or multifilaments of polypropylene or of polyester.
Patent application PCT WO-A-97/02789 describes an open knit based on a monofilament of a biocompatible polymer material, the pattern of said knit defining a plurality of cells having a substantially polygonal shape. The fabric is intended in particular for the repair of hernias, and is based on a polypropylene monofilament, of diameter 0.15 mm, whose knitting pattern is defined by two laps of yarns knitted together according to the following scheme:
front lap: 2/0-2/4-2/0-4/6-4/2-4/6;
rear lap: 4/6-4/2-4/6-2/0-2/4-2/0.
The purpose of the invention described in this document is to provide a fabric with a better transparency in order to facilitate its use in laparoscopic repair techniques. Furthermore, it is mentioned, in page 1, lines 31-32 of the published application, that the knit must have the physical characteristics and performance of conventional repair prosthetic fabrics.
However, such conventional fabrics have several disadvantages, related to the very structure of the fabric, or to the choice of the basic material used for forming the yarns, or else to the way in which the prosthetic fabric is manufactured. Thus, the mechanical properties of the conventional fabrics obtained are extremely variable from one fabric to another depending on the orientation of the specimen: vertical (also called production direction or columns) or horizontal (also called cross direction or rows). By way of example, the knitting scheme used in document WO-A-97/02789 makes it possible to obtain a knit which is about two times stronger in weft than in warp, and this corresponds entirely with the properties of conventional fabrics. A recent publication compares the mechanical properties of the principal fabrics used up to the present time (cf. Table 1 below). The differences range from 1 to 40 for the tear strength and from 1 to 8 for the tensile strength. The values of elongation under physiological loads or at break are rarely mentioned (even less so in the diagonal direction), even though they are an important parameter for the functionality of this type of product.
Source: U. Klinge et al: Verxc3xa4nderung der Bauchwandmechanik nach Mesh-Implantation. Langenbecks Arch Chir (1996) 381:323-332.
Furthermore, it is also known that the tissual reaction to an implant is the result of a normal cicatrization process. Any tissual surgical traumatism is followed by a cascade of physiological events, the principal timings of which are as follows:
t0: surgical traumatism, capillary breakage;
t0 plus a few minutes: coagulation, fibrinous network, release of chemotactic factors;
t0 plus 12 to 48 hours: polynuclear dominant luecocytic afflux;
t0 plus 4 to 8 days: fibroblastic afflux;
t0 plus 24 hours to 5 days: macrophagic dominant leucocytic afflux;
t0 plus 8 to 21 days: conjunctive differentiation of the cicatricial reaction;
t0 plus 15 to 180 days: cicatricial remodeling on contact with the implant.
Even though in certain cases the exact mechanisms are as yet unknown, particularly with regard to the determinism of the intensity of the reaction, it therefore appears that the first 8 days are determinant since they condition the fibroblastic afflux.
In non-bony tissues, the equilibrium of the reaction leads to the formation of a fibroconjunctive membrane which constitutes the interface between the implanted material and the surrounding healthy tissue. Whatever the type of implant may be, the zone under the direct influence of a biocompatible conventional material is a minimum of about 50 micrometers.
Furthermore, in the treatment of parietal inadequacies (principally hernias and eventrations), the prosthetic fabric has the task of providing additional mechanical strength to the surgical reconstruction, and its effectiveness and its local tolerance are directly related to the degree to which its tissual integration is intimate and early.
The Applicant has established by its own studies that several aspects influence tissual response to a prosthetic implant:
the constituent material of the prosthesis and its possible products of degradation in a biological medium must not induce any effect which is toxic or contrary to the sought effect. In the case of a prosthetic fabric implanted for long-term use, the aging of the material and its consequences (wear, expulsion of components, etc.) is the most difficult factor to anticipate; only raw materials validated over a long period provide maximum safety;
in a manner of speaking, as the organism sees only the surface of the material, the properties of the latter are of significant importance. Among all the surface parameters, the surface energy and the roughness play an important role. In fact, when it is sought to promote cellular integration, the surface must be absorbent (high surface energy) and smooth on a cellular scale (of the order of one micron);
only the porosity accessible by the organism is useful with regard to the anchoring of the prosthetic fabric. This porosity must be interconnected in a given volume, and there must be sufficient space for a significant cellular penetration (of the order of about 20 to 80 micrometers), and for a tissual differentiation (100 to 300 micrometers generally constitute a minimum for complete differentiation). It has been recalled previously that the minimum thickness of the tissual reaction is of the order of 50 micrometers, which means that, for porosity sizes less than 100 micrometers, the rehabilitation tissue will be entirely under the influence of the presence of the implant with little possibility of complete tissual differentiation occurring;
vascularization and the biomechanical environment of the receiving site condition the intensity of the tissual response. A richly vascularized site (skin, muscles, etc.) will react faster and more intensely than less vascularized tissues (forward chamber of the eye, bone, etc.). Furthermore, the very nature of the receiving site conditions the capacity of identical regeneration of the wounded tissue. Bone, connective tissues, mucous membranes, certain parenchymes (the liver for example) can regenerate identically without significant fibrous cicatrice. On the other hand, other very specialized tissues (muscles, nerve tissues, etc.) have lost all capacity of regeneration, the cicatrization of these tissues therefore takes place only by fibrosis;
surgical traumatism constitutes one of the principal triggering factors of the previously described cascade reaction. The greater it is, the more intense will be the reaction and the more pronounced will be its consequences (cicatrization time, fibrous after-effects, pain, etc.);
reinforcement becomes more effective and its local tolerance becomes better if its tissual integration is intimate and early. In order to be intimate and early without the formation of peripheral fibrous shell, the macroporosities of the implant must, as widely as possible, be open to the exterior and the elasticity of the reinforcement must allow it to follow the physiological deformations of the wall. The upper limits being given by the mechanical strength ( greater than 10 Kg in the standard NFG07-001 test) of the fabric, the ability to be manipulated by the surgeon and the impossibility of hernia recurrence through the pores of the fabric of maximum diameter 5 mm).
Considering the above, one of the objectives of a good prosthetic fabric is a tissual integration which is as fast as possible, providing a mechanically satisfactory anchoring without extensive fibrosis, which is a source of discomfort and pain.
The present invention describes a new type of knitted reinforcement providing isotropic elastic mechanical properties, that is to say substantially equivalent in all directions whilst retaining high porosity for a better tissual rehabilitation and sufficient rigidity for satisfactory surgical manipulation.
The fabric in question is an isoelastic prosthetic open knit based on a monofilament of a biocompatible polymer material, whose pattern is defined by a front lap and a rear lap of yarns knitted together and whose pattern determines a plurality of cells each having a substantially polygonal shape.
According to the invention, the pattern gives the knit a multidirectional tensile behavior such as obtained by a front lap capable of being obtained by knitting according to a scheme 5-4/4-3/2-1/0-1/1-2/3-4 and by a rear lap capable of being obtained by knitting according to a scheme 0-1/1-2/3-4/5-4/4-3/2-1.
The applicant has discovered that a prosthetic knit based on a biocompatible monofilament, such as defined above, has the following advantages:
the knit is flexible and not very dense in material, the weight per m2 being reduced, for example by at least 30%, in comparison with equivalent products on the market, which makes it possible to obtain better local tolerance;
the knit has sufficient firmness (spring effect during imposed deformations) for easy manipulation by the surgeon;
the knit cannot become unraveled whatever cuts may be made by the surgeon, with few filament drops along the cutting line, unlike the known fabrics;
the isoelasticity, illustrated by relatively linear slopes of stress/strain curves according to the direction of tension of the knit, makes it possible to obtain biomechanical adequation with the abdominal wall;
the knit is relatively transparent for better visibility of the covered anatomical structures in particular during laparoscopic positioning;
the knit is porous with principal openings of the order of one millimeter for better cellular penetration and in-depth conjunctive differentiation without peripheral fibrous shell phenomena, a potential source of discomfort and pain.
In a preferred embodiment of the invention, the knit has a tensile behavior defined by a ratio of measurable values, for example the tensile strength, in the vertical/horizontal/diagonal stress directions, of between 0.9:1:1.5 and 1.5:1:1.5.
Preferably, the knit has at least any one of the following properties:
the stress at break of the knit according to the NFG07-001 standard is of the order of 25 daN in all directions;
the elongation at break of the knit according to the NFG07-001 standard is of the order of 70% in all directions;
the initiated-tear strength of the knit according to the NFG07-001 standard is of the order of 5 daN;
the elongation under stress of 10 daN of the knit according to the NFG07-001 standard is of the order of 35% in all directions.
Preferably, the open knit is based on a monofilament of biocompatible polymer material chosen from the group consisting of polypropylene, polyester and polyamide, and preferably is a polypropylene monofilament.
Preferably, the diameter of the monofilament is between 0.12 mm and 0.18 mm, and preferably is 0.15 mm.
Preferably, the pattern of the knit forms unblocked open meshes defining principal cells surrounded by cells of smaller size, favoring rehabilitation.
Advantageously, each principal cell has a diameter of from about 1 mm to about 2 mm, and preferably a diameter of between 1.5 mm and 1.9 mm, whilst the smaller cells have a diameter of between 0.4 and 0.8 mm and preferably of between 0.6 and 0.7 mm.
The thickness of the knit is between 0.5 mm and 1 mm, and preferably is at most equal to 0.7 mm.
Preferably, the knit has a weight per unit area of between 50 g/m2 and 100 g/m2, and preferably a specific weight of about 75 g/m2.
Another subject of the invention is a parietal and/or visceral prosthesis, obtained with a knit such as defined above.
Yet another subject of the invention is the use of a knit according to the invention such as defined, in order to obtain a prosthetic product for surgical use, in particular for manufacturing a parietal and/or visceral prosthesis.